This paper presents a line commutated converter (LCC) based multi-terminal HVDC transmission (MTDC) system feeding very weak AC networks with hybrid reactive power compensators (RPC's) at the inverter AC side. The hybrid compensator is accomplished by the equal mixing of any two of the following compensators: synchronous compensator (SC); static var compensator (SVC); static synchronous compensator (STATCOM). The four-terminal HVDC transmission system model is implemented in the Matlab with the

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... firefly algorithm based optimal proportional integral (PI) controller for rectifiers and inverters control. The transient performances of hybrid RPC's (SC+SVC, SVC+STATCOM and SC+STATCOM) are studied under various fault conditions and the results are compared with the performance of the SC, SVC and STATCOM to focus the high quality of the hybrid compensators. The simulation results authorize that the equivalent mixture of SC and STATCOM has a steady and fastest response. The results also reveal the supremacy of the firefly algorithm based optimal PI controller over the conventional PI controller. The harmonic present in the inverter side AC quantities is also calculated under steady state operation to assure the quality of power supply.